Wide Band Spectrometer for HIFI
Wideband Acousto-Optical Spectrometer (AOS) have become a standard tool in radioastronomy for spectroscopic observations in the mm/submm frequency range. Many observatories like e.g. AST/RO, KOSMA, CSO or SEST are usingsuch spectrometers exclusively since several years. Because of the relatively simple design of an AOS itis a very suitable instrument for space applications, and the recent developments of AOS for SWAS with 1.4 and ODIN with 1 GHz bandwidth have demonstrated that this technology is mature enough for space applications in general. This includes applications in radioastronomy as well as in atmospheric research for example. The figures for power consumption, weight, or volume show that other technologies like digital correlators or filterbanks do not yet provide comparable performance, at least for large bandwidth applications in the GHz range and large numbers of frequency channels at the same time. For Herschel the requested instantaneous frequency coverage in the HIFI instrument is 8 GHz intotal, the frequency resolution should be about 1 MHz. This means that approximately 16,000 frequencypixels for full Nyquist sampling are required, which presently only acousto-optics can provide atreasonable effort.
The maximum bandwidth of acousto-optical deflectors is limited due to the rather strong acousticattenuation in the crystal materials at higher frequencies. This limits presently the maximum usablefrequency which can be efficiently processed in a Bragg-cell to about 3 GHz, if a resolution of 1 MHz isassumed. Therefore, 1.5 GHz is approximately the maximum bandwidth of an AOS at higher frequencyresolution. If a larger IF bandwidth needs to be processed, a hybrid solution is the only choice. The HIFIinstrument is planned for 4 GHz IF bandwidth, therefore 4 times 1 GHz can be used for full frequencycoverage. This leads, when operating 2 mixers simultaneously, to the fabrication of 8 individual AOSbands in total. This high number of back-ends is presently not available at any of the ground-basedobservatories around the world. Therefore there is very significant technological effort required for thedevelopment of the HIFI back-end system.
For us, the Array-AOS seemed to be the logical next step in development in order to serve the needs offuture array-receiver programs in several observatories. A few years ago, the development was started inorder to make a rather cheap and very reliable back-end solution available.
WBS Breakdown Structure
The following scheme shows the breakdown structure of the WBS. Klick into the WBE/I and WBO boxes to get more specific information about these units.